Chemistry Paper 2 (Essay) ,May/June 2010

Questions:		1	2	3	4	5	6	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 3
(a) (i) Define a base according to Arrhenius concept. (ii) Give one example of an Arrhenius base. (iii) Identify each of the following substances in aqueous solutions as strong electrolyte, non-electrolyte or weak electrolyte. I. C_I2H₂₂0₁₁. II. NH₃ III. NaOH (iv) Write a balanced equation to represent the reaction between CH₃COOH and KOH [ 8 marks] (b) Calculate the volume of 0.500 mol dm-3 HCl required to-neutralize 20.00cm of 0.300 moldm-3 NaOH. [ 3 marks] (c) Give the IUPAC name of each of the following salts: (ii) NaOCl; (iii) Mg(HC0₃)2 [ 2 marks] (d) (i) Define the term standard solution (ii) Consider the following compounds: NaOH and Na2C0₃ Which of the compounds is suitable for the preparation of a standard alkaline solution? Give reason for your answer. (iii) Fe completely reacted with dilute HCl. 1. Write an equation for the reaction. II. If 3 .08 g of Fe completely reacted with 50.0cm3 of 2.20 moldm-3 HCl, calculate the relative atomic mass of the metal. [ 12 marks]
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OBSERVATION
The question was attempted by most candidates and the performance was fair. In(a)(i) and (ii), candidates correctly defined Arrhenius base as a substance which produces hydroxyl (OH-) ions in its aqueous solutions and gave one example from among NaOH, Ca(OH)2, KOH and NH40H (names were also accepted for the examples). In(a)(iii),only few candidates correctly identified aqueous solutions of: I. C_I2H₂₂0₁₁. as non - electrolyte; II. NH3 as weak electrolyte; III. NaOH as strong electrolyte. In(a)(iv), most candidates could not write a balanced equation to represent the reaction between CH3COOH and KOH. The expected answer from candidate was as follows. In part(b), candidates correctly calculated the volume ofHCl required for complete nuetrilization of the NaOH thus: CAVA = 1 CBVB VA = CBVB = CA 0.30 X 20 0.50 = 12.00cm3 (other method was accepted) In parte c )(i) and (ii), candidates correctly gave the IUP AC name of each salt as sodium oxochlorate (1) and magnesium hydrogen trioxocarbonate (IV) respectively. In parte d)(i), candidates correctly defined standard solution as a solution whose concentration is (accurately) known. However, most of them could not state the compound which is suitable or the preparation of a standard alkaline solution nor give reason for their answer in (d)(ii). The expected response from candidates was Na2C03, because it is stable in air/non deliquescent. In(d)(iii), candidates correctly wrote an equation for the reaction between Fe and dilute HCl in I and calculated the relative atomic mass of the metal in the given reaction in II as follows: 1. Fe(s) + 2HCl (aq) --+ FeCl2(aq) + H2(g) II. Number of moles of HCI used = 2.20 x 50.0 1000 = 0.11 Mole ratio ofHCl: Fe = 2: 1 :. number of moles of Fe used = 0.11 = 0.055 mole 2 0.055 mole of Fe = 3.08g :. 1.0 mole of Fe = 3.08 x 1.0 0.055 = 56g Relative atomic mass of Fe = 56 (other method was accepted)